The typical way to do this is to use the "Caller System Transfer" conversation for this - in the caller input section you can map a one key rule to go to that conversation and the caller will be allowed to "Free dial" an extension and do a release transfer to that number.
The "Default System Transfer" restriction table determines which numbers are allowed here - by default all numbers are blocked so you need to go to that restriction table and open it up for dialing - presumably you'll want to limit it to, say, 5 digit extensions and no allow 9 as the first digit etc... so folks are calling long distance numbers on your dime.
The somewhat more advanced way is to edit the greeting rule for a call handler (i.e. the opening greeting call handler's standard greeting) and on the basic settings page check the "Allow Transfers to /Numbers not associated with users or call handlers" - this allows the user to free dial directly from that greeting and if the number does not match an extension of a user/handler in the system AND the number is allowed by the "Default System Transfer" restriction table, Connection will do a release transfer directly to that number. So if callers are savvy enough to know what number they want to dial and need no prompting, this works well.
